The present invention concerns a piezo-electric elongation sensor for measuring positive and negative elongations on surfaces of rigid structures by means of electrical signals, where the elongation sensor can be pressed in a force fit onto this surface or permanently attached to it. The invention also concerns an elongation detector with at least one elongation sensor and a process for electrical measurement with an elongation sensor or elongation detector.
It has long been known that on application of a force to a piezo-electric body, due to the dielectric shift, surface charges occur which are detected with electrodes. It is also known in press and tool construction but also on other rigid structures to use piezo-electric elongation sensors as surface elongation sensors. On these rigid structures and on robust machine tools, elongation sensors with a high degree of resolution are required. Elongation sensors with high sensitivity are also required for monitoring parts clamped between two tool halves and not removed from the mould, as for example on a 1000 kN press up to 50 N must be detected. In this order of magnitude one microstrain (xcexcxcex5) must be able to emit a 10 Volt signal (V). No elongation sensors or corresponding processes known today are able to do this.
In contrast elongation detectors are known which are pressed onto the surface to be measured by means of two screws, so that elongations occurring axially between the two screws can be measured. By means of mechanical amplification the surface elongation can be amplified for example by up to a factor of 10. Such detectors as indicated have the disadvantage that only axial elongations between the two screws can be detected and the sensitivity does not achieve the required resolution.
Piezo-electric action sensors are also known which are pressed onto the surface to be measured by just one screw. Here two integrated piezo-quartz discs are loaded with thrust forces, the electric charges generated can be detected. These elongation sensors however give too low a resolution. CH,A5 687647 describes detectors with elongation-sensitive elements, i.e. elongation sensors, which can include piezo films. According to this Swiss patent specification also, because of the choice of measuring elements it has not been possible to measure with the high resolution required for special applications for example in the press, tooling and machine tool industries.
Furthermore, all sensors stated above must be fitted direction-dependent, in other words to achieve optimum measurement results the main elongation direction must always be found. In most cases however this involves great extra cost.
The present invention is therefore based on the task of obtaining, with a piezo-electric elongation sensor of the type stated above, direction-independent elongation measurements with high resolution. The elongation sensor can be used in an optimally adapted elongation detector using an optimum process.
With regard to the elongation sensor, the task is solved according to the invention in that an elongation sensor, highly sensitive in all elongation directions, consists of a piezo-ceramic material which is applied as a thin layer on at least one side of a flexible metal plate or permanently attached to this as a moulded body. Special and further design forms of the elongation sensor arise from the dependent claims.
In the elongation measurement according to the invention, there is no direct force action on the piezo-ceramic material as is otherwise the case. In metal-metal-ceramic connections, tangential forces as a result of positive or negative elongations are transferred from the surface to be measured to the metal plate and thence to the elongation sensor where they create surface charges which can be detected. Thanks to the replacement of the previous conventional piezo-electric materials by piezo-ceramics, even the smallest elongations generate such large electrical charges that they can be evaluated directly or suitably with charge amplifiers.
Piezo-ceramic materials are known in themselves, these are usually oxidic materials based for example on lead oxide, zirconium oxide and titanium oxide. By the addition of further metal oxides for example oxides of the elements lithium, magnesium, zinc, nickel, manganese, niobium, antimony and/or strontium, the material parameters can be finely matched and stabilised. Furthermore the physical properties of the piezo-ceramic materials can be influenced by varying the mixing ratio of the base materials, the grinding duration of the components, the sintering conditions and the forming.
Lead-zirconate-titanate, known in itself as a piezo-ceramic is preferably used according to the invention. The crystal structure of lead-zirconate-titanate is derived from perovskite, a mineral with chemical formula CaTiO3.
The carrier for the piezo-ceramic material preferably consists of a metal disc punched out of a steel or brass plate of a thickness of suitably 0.05 to 0.25 mm and with a diameter in the range of 10 to 60 mm. The piezo-ceramic material is permanently attached coaxially, preferably with a diameter of 5-30 mm and a thickness of suitably 0.05 to 0.25 mm. This can be achieved firstly by gluing or soldering a preshaped ceramic disc or secondly by chemical deposition from the gaseous phase, a process known as CVD (chemical vapour deposition), electrochemical deposition or vaporisation on the metal plate. The adhesion strength of the piezo-ceramic on the metal disc, however it is applied, must fulfil high requirements. Bending around a round bar of 10 mm diameter must not lead to any separation between the piezo-ceramic and the metal disc.
According to a particularly advantageous embodiment of the invention, the elongation sensors are formed as piezo-ceramic transducer membranes. These have a mechanical fundamental resonance which is defined by the geometry. In addition to the fundamental resonance, harmonic oscillations and resonances occur in the longitudinal and transverse direction, for discs in particular also in the radial direction if the diameter is essentially greater than the thickness.
With an elongation sensor according to the invention for example the following technical data can be achieved:
The piezo-ceramic elongation sensor according to the invention can also be used for monitoring tasks on very rigid fast-running presses to detect process faults, for example in the form of seized parts. Furthermore the elongation sensor can be used for mould protection in injection moulding and metal diecasting tools by being pressed on or attached at an optimum point on the machine structure, for example on a toggle lever. Deviations from the ideal closing curve can be measured with high resolution according to the invention. Thus 5 kg can be detected on a 100 t press. Despite this, a 500 times greater elongation can also be detected using the same elongation sensor.
The piezo-ceramic elongation sensor can be pressed onto a surface to be measured with any suitable resilient contact body until a force or friction fit is achieved between the metal plate and the surface. The metal plate can also be glued, soldered or permanently connected in another suitable manner to the surface to be measured. In all cases positive and negative elongations are transferred from the surface to the metal plate of the elongation sensor and from this to the piezo-ceramic material. This piezo-electric measurement principle allows resolutions in the nanometre band.
A particularly advantageous elongation detector with at least one piezo-ceramic elongation sensor is characterised in that the elongation sensors are arranged on the free face of at least one resilient pressure body inlet with a projection into a detector housing which is stable in itself, and comprises means for pressing the detector housing onto the surface to be measured.
This detector housing is preferably penetrated by a screw hole or has at least two flanges flush with the opening and evenly distributed over the periphery, each with a screw hole or slot. The detector housing which is stable in itself thus allows a strong pressure of the elongation sensor onto the surface to be measured by way of the resilient pressure body.
Suitably the elongation sensors on the projecting pressure body of the elongation detector are covered with a metallic or metallised film and are thus protected from harmful chemical and/or mechanical effects. This film can be removed for installation of the elongation detector, preferably the elongation detector is attached with the protective film applied.
In relation to the process for electric measurement with an elongation sensor or elongation detector, the task is solved according to the invention in that the piezo-ceramic emitted charges are amplified in a charge amplifier into output signals which can be used by controls and which have a voltage proportional to the charge, where the charge amplifier switches very quickly from small to very large charge bands if necessary.
The high sensitivity of the elongation sensors or elongation detector according to the invention, connected with the high measurement band of up to 500 microstrains, requires immediately a charge amplifier, known in itself, with a very fast band change in the microsecond range, from an extremely small measurement band to a large measurement band. Thus for example with one and the same elongation sensor both high sensitivity mould protection and total force measurement can be achieved.
Charge amplifiers convert the charge emitted by the elongation sensor into a voltage proportional to this. This is usually achieved in that the charge emitted by the elongation sensor creates, at the inverting input of an operational amplifier, an inversely proportional current in a feedback branch. If the current in the feedback branch is allowed to flow through a capacitor, at the output to the operational amplifier a voltage proportional to the sensor signal is generated. As the insulation resistance is not infinitely high and the fault currents of the operational amplifier are not zero, over time at the output of the operational amplifier a voltage perceptible as a residual deflection occurs, the so-called drift. It is therefore necessary to reset the output voltage from time to time or discharge the capacitor fully in the feedback branch.
The output voltage is firstly proportional to the sensor signal and secondly dependent on the capacitance of the capacitor in the feedback branch. This capacitance determines how much charge is required to achieve a particular voltage value. It is therefore known as a band capacitor.
To produce a charge amplifier with two or more measurement ranges, the output voltage is amplified by the required factor. This gives a more sensitive measurement band in the simplest manner.
It is reasonable that the capacitance of the band capacitor must be adapted to the largest measurement band required, as by amplification only smaller measurement bands can be implemented. As long as the amplification factor is not selected too high (typical values are factors from 2 to 10), the drift is not important as this too is amplified by the relevant factor. If however the charge amplifier is to be produced with bands lying far apart, this solution is no longer likely to be successful.
For large band differences, a circuit variant with a switchable band capacitor must be selected. Here the bands can stand in any ratio to each other and the limits are set only by the stepping of the capacitance by the manufacturer.
According to a special design form of the process, to eliminate mechanical faults at high resolution measurements initially an ideal curve is recorded and stored. Each further cycle is monitored and compared in real time with the ideal curve. When a fault occurs in real time, the ideal curve is subtracted from the fault curve, the difference compared directly with an alarm threshold and the fault detected as an alarm threshold value.
In contrast to processes previously known for elongation measurement, all elongations occurring on the surface can be detected direction-independently. The complex and restrictive definition of a main direction can be omitted.
The measurements on the surface can be carried out not only with pressure sensors pressed on to create a force fit. The metal plate of the elongation sensor according to the invention can be glued or soldered to the surface to be measured. This must be done with such high quality that the smallest elongations, in the nanometre band, are transferred to the piezo-ceramic material.